Hard pass.

OP, please take this advice not as an attack. I am genuinely concerned and want to give my honest thoughts. Your bestfriend should know her limits. If your girlfriend told you her thoughts and opinions about this honestly, it’s likely because she feels iffy about it. It’s good that she even tags along when your BFF needs help. I don’t think there’s anything wrong with having a bestfriend, but there’s gotta be a limit to everything. Allowing her to do these things is like saying they are equal in your eyes. Forehead kissing, arm-resting, snuggling up.. those things may not be sex but they are intimate, even more intimate than sex if you ask me. If that’s the case, why even have a GF? Hope it works out for you.